This invention pertains to the art of sensors and more particularly, to contact type sensors used for monitoring or counting sheet products.
The invention is particularly applicable to counting paper products such as newspapers and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications and may be advantageously employed in other sheet product environments and applications.
Two general categories of paper counters are utilized in the industry. A mechanical or contact type counter is relatively inexpensive. Typically a sprocket or star-shaped wheel engages a leading or front edge of each newspaper in a lapped stream as it passes by the mechanical counter. Particularly, distinct lobes of the rotary, mechanical counter engage successive newspapers in the lapped stream. Rotation of the counter is monitored to indicate the number of newspapers that have passed.
Even though, as indicated above, these mechanical counters are relatively inexpensive, they are deemed ineffective and result in wide variations of efficiency. Particularly, mechanical counters are not generally applicable to product streams comprised of single sheet papers or other small thicknesses of papers due to the limited height differential between adjacent papers. Additionally, papers having limited thickness will not impose a substantial enough force on a lobe to register a "count". Rather than actuating the lobes of the star-shaped wheel, the papers may become dislodged from their imbricated arrangement resulting in bunching of the papers.
Mechanical counters necessarily count the leading or folded edge of the paper and in some situations it may be preferable to count the trailing edge. That is, the papers must be oriented so that the leading edge of each paper is exposed for contact with the mechanical counter. Typically, the papers are disposed on a conveyor surface with the lobes of a mechanical counter extending toward the conveyor surface and into the path of the papers. For example, with a selected paper, the trailing edge of a next adjacent, downstream paper is received underneath the leading edge of the paper. The trailing edge of the paper is received underneath the next adjacent, upstream paper so that the leading edges are disposed for contact with the mechanical counter. Even with these drawbacks, the mechanical counter is widely used in the industry.
A more recent introduction to the industry is a laser type counter. The laser type counter is deemed to be more reliable and effective than a mechanical counter but is also very expensive due to its complexity. Even then, the laser type counter is susceptible to problems with color and/or ripples in the sheet product. That is, if the sheet product has ripples the laser will provide additional counts because of the deviation from a planar, "standard" sheet product conformation.
Still another problem associated with laser type counters is the ineffectiveness that results from certain types of sheet products. For example, newspapers that have large pictures on a page facing the laser sensor rather than substantial text material are often miscounted. It is believed that such errors result from the pictures absorbing the light and providing insufficient feedback to the laser sensor to adequately count the sheet products.